The present invention relates to a light-exposure apparatus used in semiconductor fabrication, more particularly to the arrangement of the shutter and filters of the apparatus.
FIG. 1 schematically shows the structure of a conventional light-exposure apparatus 50 having a light source such as a mercury lamp 51, and a shutter 52 mounted just below the light source. The apparatus 50 has three operating states: a shut state, an alignment state, and an exposure state. FIG. 1 illustrates the light path in the alignment state.
The shutter 52 moves between a shut position (indicated by dotted lines) in which it blocks the light from the mercury lamp 51, and an open position (indicated by solid lines) in which the light is not blocked and can be used for alignment and exposure purposes. The light is refracted by the transmitting part 53a of a convex secondary lens 53, reflected by a primary mirror 54, then reflected by an aluminum coating on the reflecting part 53b of the secondary lens 53. After passing through a slit 55, the light is reflected by the reflecting surface 56a of a toroidal mirror 56, then passes through one of two filters, either an exposure filter 57 that transmits light of comparatively short wavelengths, or an alignment filter 58 that transmits light of longer wavelengths. In the alignment state, these filters are positioned so that the light passes through the alignment filter 58, as shown.
The elements described so far constitute the illumination system 65 of the apparatus. The light emerging from the illumination system 65 is further reflected by three relay mirrors 59, 60, 61, then illuminates a semiconductor wafer through a mask. The wafer and mask are not shown in FIG. 1.
The wafer is coated with a photoresist material that is insensitive to the wavelengths transmitted by the alignment filter 58. This light can therefore be used for alignment of the wafer and mask. After alignment is completed, the exposure filter 57 is moved into the light path, replacing the alignment filter 58, and light of a shorter, more energetic wavelength is used to transfer the mask pattern to the photoresist.
In the shut state, the shutter 52 is moved to the position indicated by dotted lines to block the light from the mercury lamp 51, so that the light does not reach the secondary lens 53, primary mirror 54, slit 55, and toroidal mirror 56 in the illumination system 65.
A problem in the conventional apparatus 50 is that the short-wavelength light that interacts with the photoresist also interacts to some extent with the optical elements in the illumination system 65, gradually clouding or darkening the coatings on their surfaces, for example. Since these optical elements are exposed to all emitted wavelengths in both the exposure state and the alignment state, they receive continuous exposure to short-wavelength light during these two states. As a result, these optical elements, more specifically the secondary lens 53, primary mirror 54, and toroidal mirror 56, tend to degrade comparatively quickly.
Continuous exposure to the light emitted by the mercury lamp 51 during the alignment and exposure states also raises the optical elements in the illumination system 65 to a comparatively high temperature. In the shut state, in which the shutter 52 is closed, these optical elements receive no illumination, and their temperature falls back toward room temperature. As the apparatus 50 cycles repeatedly among the shut, alignment, and exposure states, the optical elements in the illumination system 65 undergo repeated large temperature swings, which affect their optical properties and contribute to the degradation thereof.
An object of the present invention is to reduce the degradation of optical elements in an apparatus for projecting a mask pattern onto a wafer.
A further object is to reduce temperature variations in the illumination system of the apparatus.
The invented apparatus has an illumination system providing light for use in alignment and exposure, and a projection system using the provided light to project a mask pattern onto a wafer. The illumination system has a light source, two light selectors, and at least one optical element disposed between the two light selectors. The first light selector has an alignment filter, which it selectively places between the light source and the optical element, in the path of the light emitted by the light source. The second light selector has at least an exposure filter and a shutter, which it selectively places between the optical element and the projection system, in the path of the light received from the optical element.
The apparatus can be operated in a shut state, an alignment state, and an exposure state. The alignment filter is placed in the light path in the shut state and the alignment state, and is removed from the light path in the exposure state.
Degradation of the optical element is reduced because the optical element is exposed to unfiltered light only in the exposure state, and not in the alignment state.
Temperature variations are reduced because the optical element is illuminated in the shut state, as well as in the alignment state and exposure state.